**3.1 Rheological measurments**

A rotational rheometer RheolabQC coupled with Rheoplus software (Anton Paar) was used for different reactor fluids, which recorded the rheograms´ and allowed subsequent data analysis. The temperature was maintained constant at 370.2 °C. The reactor fluid volume used for each measurement was 17 ml. Reactor fluids from mesophilic (37°C) lab-scale reactors (4 L running volume), with a hydraulic retention time (HRT) of 20 days, were sampled.

Five lab-scale reactors (A-E) were sampled before the daily feeding of substrates. All reactors had been running for at least three HRTs prior to sampling. The different substrates treated were slaughter household waste, biosludge from pulp- and paper mill industries, wheat stillage and cereal residues. The TS values ranged between 3.1−3.9 % for four of the reactors while one was at 7.7 % (Table 1).


Table 1. Fluids from five lab-scale reactors were chosen for rheological measurements. A short description of their TS values and substrates are presented.

Rheological measurements were carried out with a three-step protocol where (1) the shear rate increased linearly from 0 to 800 s-1 in 800 sec., (2) maintaining constant shear rate at 800 s-1 in 30 sec, (3) decreasing linearly the shear rate from 800 to 0 s-1 in 800 sec., according to Björn *et al*. (2010). For each sample three measurements were carried out and performed immediately after sampling or stored at +4 ºC pending analysis.

The fluid behaviour was interpreted by the flow- and viscosity curves according to Schramm (2000), and the dynamic viscosity, limit viscosity and yield stress were noticed. The three most common mathematical models for non-Newtonian fluids; Herschel Bulkley model; Ostwald model (Power Law) and Bingham model, were applied in order to transform rheogram data values to the rheological behaviour of the fluids. Flow behaviour index (n) and consistency index (K) were studied.
